1. Field of the Invention
The present invention relates to precision mirror fabrication, and more specifically, it relates to techniques for adjusting the figure of a substrate for a precision mirror.
2. Description of Related Art
Present methods for fabricating precision mirrors are very time consuming and costly. The final figure and finish is obtained using many iterations and a precision measurement of the figure is required after each polishing step.
In U.S. Pat. No. 5,957,749, titled “Apparatus For Optical Inspection Of Wafers During Polishing” an optical system is disclosed for the inspection of wafers during polishing which also includes a measurement system for measuring the thickness of the wafer's top layer. The optical system views the wafer through a window and includes a gripping system, which places the wafer in a predetermined viewing location while maintaining the patterned surface completely under water.
U.S. Pat. No. 4,018,638, titled “Method Of Reducing The Thickness Of A Wafer Of Fragile Material” is method of reducing the thickness of a wafer of fragile material, e.g., pyroelectric material, by placing the wafer, supported only at its rim, in a holder filled with a non-corrosive liquid.
U.S. Pat. No. 4,793,895, titled “In Situ Conductivity Monitoring Technique For Chemical/Mechanical Planarization Endpoint Detection” discloses an apparatus and method for monitoring the conductivity of a semiconductor wafer during the course of a polishing process.
The invention disclosed in U.S. Pat. No. 5,081,421, titled “In Situ Monitoring Technique And Apparatus For Chemical/Mechanical Planarization Endpoint Detection” provides an in situ monitoring technique and apparatus for chemical/mechanical planarization end point detection in the process of fabricating semiconductor or optical devices. The detection in the present invention is accomplished by means of capacitively measuring the thickness of a dielectric layer on a conductive substrate.
In U.S. Pat. No. 5,125,740, titled “Method And Apparatus For Measuring Optical Constants Of A Thin Film As Well As Method And Apparatus For Fabricating A Thin Film Utilizing Same”, a sample is located so as to be close to a prism and a light beam coming from a light source is projected to the prism while varying the incident angle to the prism as a parameter. Optical constants such as the refractive index, the film thickness, the distribution of the refractive index, etc. are obtained by calculation, starting from measured values thus obtained.
In U.S. Pat. No. 5,157,877, titled “Method For Preparing A Semiconductor Wafer”, the polishing of a semiconductor is effected by a method which comprises preparing a finished backing pad by the precision surface machining operation, setting the semiconductor wafer on a wafer holding jig having a template containing at least one wafer-positioning hole fixed on a carrier plate in such a manner that the backing pad enters the positioning hole, and polishing the semiconductor wafer.
U.S. Pat. No. 5,240,552, titled “Chemical Mechanical Planarization (Cmp) Of A Semiconductor Wafer Using Acoustical Waves For In-Situ End Point Detection” describes a method and apparatus for chemically mechanically planarizing (CMP) a semiconductor wafer.
U.S. Pat. No. 5,293,216, titled “Sensor For Semiconductor Device Manufacturing Process Control” describes a fiber-optic sensor device for semiconductor device manufacturing process control that measures polycrystalline film thickness as well as surface roughness and spectral emissivity of a semiconductor wafer.
U.S. Pat. No. 5,337,015, titled “In-Situ Endpoint Detection Method And Apparatus For Chemical-Mechanical Polishing Using Low Amplitude Input Voltage”, discloses an in-situ thickness monitoring/endpoint detection method and apparatus for chemical-mechanical polishing (CMP) of a dielectric layer on a top surface of a semiconductor wafer.
U.S. Pat. No. 5,433,651, titled “In-Situ Endpoint Detection And Process Monitoring Method And Apparatus For Chemical-Mechanical Polishing” discloses an in-situ chemical-mechanical polishing process monitor apparatus for monitoring a polishing process during polishing of a workpiece in a polishing machine.
U.S. Pat. No. 5,492,594, titled “Chemical-Mechanical Polishing Tool With End Point Measurement Station” discloses a wafer polishing and planarizing tool in which there is incorporated a separate measuring station and means for moving the wafer and immersing the wafer into the measuring station without removing it from the polishing head.
U.S. Pat. No. 5,657,123, titled “Film Thickness Measuring Apparatus, Film Thickness Measuring Method And Wafer Polishing System Measuring A Film Thickness In Conjunction With A Liquid Tank” provides a light interference-type film thickness measuring mechanism that measures a film thickness with light directed onto the bottom surface of a wafer held by a wafer holding head.
U.S. Pat. No. 5,658,418, titled “Apparatus For Monitoring The Dry Etching Of A Dielectric Film To A Given Thickness In An Integrated Circuit” discloses detecting the desired etch end point in the dry etching of a structure.
U.S. Pat. No. 5,719,495, titled “Apparatus For Semiconductor Device Fabrication Diagnosis And Prognosis” provides a sensor for diagnosis and prognosis of semiconductor device fabrication processes that measures specular, scattered, and total surface reflectances and transmittances of semiconductor wafers.
In U.S. Pat. No. 5,739,906, titled “Interferometric Thickness Variation Test Method For Windows And Silicon Wafers Using A Diverging Wavefront”, an interferometric apparatus and method are provided for determining a seal thickness and thickness variations of silicon wafers and other window-like optics. Thin films have been used in the past to correct the figure of mirrors by depositing thin films of the desired thickness profile on top of a substrate using evaporation masks. See W. C. Sweatt. J. W. Weed, A. V. Farnsworth, M. E. Warren, M. E. Neumann, R. S. Goeke, and R. N. Shagan, “Improving The Figure Of Very Good Mirrors By Deposition,” OSA Trends in Optics and Photonics Vol.4, “Extreme Ultraviolet Lithography”, G. Kubiak and D. Kania, Eds. Washington, D.C., Optical Soc. Of America, 1996., pp. 149-155. See also C. Tarrio, E. Spiller, C. J. Evans, T. B. Lucatorto, and C. C. V, “Post-Polish Figuring Of Optical Surfaces Using Multilayer Deposition,” ibid., pp. 144-148. However, it is time consuming and requires many iterations to produce the masks for general corrections in 2-D that is described by higher order polynomials.
As discussed above, precision mirrors are currently fabricated by using a large number of iterations between polishing/figuring and interferometric metrology of the surface figure. No data on the figure is available during polishing and the process is very time consuming. In some cases, about a day is needed in each iteration just to reach a stable temperature of the optic in the interferometer. Current methods make it very difficult to fabricate mirrors fast enough to provide for the expected number of commercial Extreme Ultraviolet Lithography (EUVL) steppers that will be needed. In order to accelerate production, it is desirable to connect metrology and polishing more tightly, ideally into a single procedure.